Frequency control



Jan. 17, 1939. N. E. LINDENBLAD FREQUENCY CONTROL Filed March 30, 1955 5 Sheets-Sheet l INVENTOR. NILS E. UNDENBLAD BY 7% g ATTORNEY.

N. E. LINDENBLAD .Fan. 17, 1939.

' FREQUENCY CONTROL 5 Shets-Sheet 2 Filed March 50, 1935 214 l/zls INVENTOR. N|L$ E. UNDENBLAD BY 74% ATTORNEY Jan. 17, 1939.- LINDENBLAD 2,143,891

FREQUENCY CONTROL Filed March 50, 1935 s Sheet-Sheet 3 Hum INVENTOR NILS' E. LINDENBLAD BY W ATTORNEY 0 QQM,

m w @SQEGMG w% y m A 7051/1/09 way l yous 7.790

Jan 17, 1939. E L'NDENBLAD 2,143,891

FREQUENCY CONTROL Filed March 30, 1935 5 Sheets-Sheet 5 PZA T 5 M00. 7055 (WIPE/ M71017 M00. 72/85 Riff/HER I ma M00. INVENTOR.

NILS E. LINDENBLAD ATTORNEY.

-1IHII Patented Jan. 17, 1939 UNITED STATES PATENT OFFICE 2,143,891 FREQUENCY CONTROL Nils E. Lindenblad, Port Jefferson, N. Y., assignoi' to Radio Corporation of America, a corporation of Delaware Application March 30, 1935, Serial No. 13,886

31 Claims.

My present invention relates to frequency modulation of short radio waves. While my frequency modulation scheme is generally'applicable to any form of short wave oscillation generator, it is particularly useful for line controlled oscillation generators. The line controlled generator which I prefer to use is one making use of a section of hollow metallic tubing of predetermined length which acts as a low loss frequency determining circuit. More detailed descriptions of such line controlled apparatus may be found in my copending application Serial No. 6,814, filed February 16, 1935 now Patent No. 2,095,990 dated October 19, 1937; in the copending application of C. W. Hansell, Serial No. 692,092, filed October 4, 1933 now Patent #2,095,980 dated October 19, 1937; and in the copending applications of F. H. Kroger, Serial No. 1,489, filed January 12, 1935 and Serial No. 5,058, filed February 5, 1935 now Patent No. 2,077,880 dated April 20, 1937.

Frequency modulation of these line controlled oscillators may be accomplished by actually changing the physical dimensions of the frequency controlling line as, for example, associating therewith a mechanically vibrating capacitive element which is caused to vibrate at a desired modulation frequency. This scheme suffers from the disadvantage that the range of frequency variation produced in the high frequency oscillator is relatively small, and the change in frequency with modulation voltage is not always linear. Moreover, the mechanically vibrating scheme may introduce an undesirable amount of amplitude modulation. It is also very limited as to frequency of modulation in that it is not fast enough for such modulation as that used in television, for instance.

To overcome these diificulties is the principal object of my present invention and in effecting it, I feed back a certain amount of energy from the output of the oscillator to its frequency controlling circuit. The phase and amplitude of the energy fed back is, however, varied by the modulating waves, as a consequence of which the capacity or inductance or both of these characteristics of the frequency controlling circuit is so varied that linear frequency modulation over a very wide range, much wider than the absolute frequency values of the modulating waves, is obtained. A particular advantage of my present invention resides in the fact that no moving parts are required, and further advantages are found in the freedom from amplitude modulation and in the linear change in the frequency of perature changes.

the high frequency waves with respect to the modulating waves, potentials or currents.

In the accompanying drawings, which are only illustrative of my present invention, and which are not to be considered in any way limitative thereof, Figure 1 illustrates a high frequency oscillator, a frequency control circuit and a modulator coupled to said oscillator and to said frequency control circuit to control the character thereof, and to frequency modulate the produced wave; Figure 2 illustrates a modification in which the grids of the modulator tubes are grounded to eliminate certain undesired capacity effects; Figure 3 is a modification wherein the inductance of the frequency controlling circuit is controlled Figure 4 is another modification in which the inductance and capacity of a frequency controlling circuit are simultaneously varied by means of the phase and amplitude controlled feed-back energy; Figure 5 is a circuit explanatory of Figure 4. Figures 6 and 7 illustrate modifications of the arrangements of Figures 1 and 2 wherein the controlling potentials are applied to the plates of the modulator tubes; while Figure 8 is a modification which includes means for compensating for undesired modulation due to modulation power variations which may occur under some conditions.

Turning to Figure 1, the grids 2, 4 of the pushpull connected oscillation generator vacuum tubes 6, 8, are coupled through the loops I0, 12 in phase opposition with each other to the quarter wave frequency controlling line [4 concentrically mounted within the outer grounded metallic cylinder Hi. The frequency controlling line system l4, IE is preferably substantially A, the length of the waves to be generated as has been more fully described in the copending applications of F. H. Kroger hereinabove referred to, and may be of the type described by Kroger in that the cylinder or tube I4 is made substantially invariable in length despite ambient tem- In fact, as a further precaution to insure frequency stability, the entire line controlling system I4, l6 may be housed within a constant temperature oven, not shown. The method of coupling the push-pull connected tubes 6, 8 in phase opposition to a quarter wave length line is more fully described and claimed in my copending application Serial No. 7,473, filed February 21, 1935. For determining proper operation of the system, the thermogalvanometer I8, capacatively coupled by the metallic stud 20 to the free end or high potential end of line, is provided. This measuring system is described and claimed more fully in my copendingapplication Serial No. 6,814, filed February 16, 1935 now Patent No. 2,095,990 dated October 19, 1937. Grid bias for the grids 2, 4 is obtained by means of the grid leak and condenser connections 22, 24.

The cathodes 26, 28 are connected together in parallel for heating currents by means of conductors 30, 32, which act cophasally for high frequency currents. This cophasal action is enhanced by means of bypassing condensers 34, 36, 38. The filament system or the cathode heating system may be structurally made as shown in my copending applications Serial No. 651,809, filed January 14, 1933 now Patent No. 2,052,888 dated September 1, 1936 and Serial No. 603,310, filed April 5, 1932 now Patent No. 2,052,576 dated September 1, 1936, and are energized by the filament heating source 40. If desired, a variable condenser 42 may be provided for adjusting the filament or cathode impedance. The plates 44, 46 of the oscillator tubes 6, 8 are connected in phase opposition through the loop circuit'48, 50 which may be made adjustable in length and tuned to the operating frequency. To assist in this adjustment a variable condenser 52 may be provided. Output energy from the oscillator may be taken from the loop 54 coupledto the cathode system or may be taken inductively from the loop 56 coupled to the anode circuit 48, 50, 52.

Voltage from the output of 6 and 8 is fed through blocking condensers 60, 62 to the grids 64, 66 of the coupling and modulating tubes 68, E0. The condensers 60 and 62 are connected to points of opposed polarity on the lines 48 and 50 so that potentials displaced substantially 180 in phase are impressed on the grids of tubes 68 and I0. The filament circuit I2 for the cathodes 14, I6 of the tubes 68, I may be made identical to that of the oscillator tubes 6, 8 and hence need not be described in detail. The plates I8, 80 of tubes 68, F0 are connected together as shown and through conductor 82 to the loop 84 tuned by means of condensers 86, 88 which, though not essential, are helpful for adjustment purposes. Plate potential for tubes 68, I0 is supplied through lead 90. The loop 84 is connected to the metallic stud 62, provided with a capacity element or plate 94 adjacent, and preferably facing the high voltage end 96 of. the quarter wave length line I4 which preferably is made hollow and cylindrical. The stud 92 may be insulatingly supported within the metallic cylinder I6 in a manner similar to the insulating support shown for stud 20, but this insulating support has been omitted for the sake of drawing clarity.

Modulating voltages are applied through the transformer I00 and radio frequency chokes I62, I04 in phase opposition to the grids of the tubes 68, I0, grid bias being fed through the secondary of the transformer I00 from the grid biasing source I06. Preferably the tubes 68, I0 are both biased to operate near the lower end of their characteristic curve. The modulating waves fed through the transformer I00 may be of audio frequencies or may be super-audible frequency waves or low radio frequency waves, as found desirable or useful.

By virtue of the phase opposition connection of the grids 64, 66 of the modulating tubes 68, I0, and the bias on said grids the phase of the energy fed back to the capacity stud 94 is varied or reversed in accordance with reversals of the modulating waves or potentials fed to the transformer I00. The amplitude of the modulating waves, which is also variable, also controls the amount of energy fed back to the modulating tubes. Hence, by virtue of this differential biasing scheme, the capacity element or stud 02, 94 carries potential of reversible phase and of varying amplitudes with respect to the end 96 of the frequency controlling line I4. These potentials of varying phase and varying amplitude oppose or add to the potentials on the stud as the relative phases of the original energy and energy fed back changes to an extent dependent upon the amplitude of the voltages fed back. Hence, there is a variation in the distribution of the electric flux from the end of the line in accordance with the modulating voltages in transformer I00 and this is equivalent to a variation in the capacity at the end of the line I4. This variation in capacity changes the natural period of the line I4 and hence the frequency of operation of the oscillator tubes 6, 0. The variation in frequency will be found to occur linearly over an extremely wide range depending upon the adjustments of the circuits used, and it will be found that the frequency modulated waves produced in the output circuits 54, 56 of the oscillator are remarkably free of amplitude modulation.

In the arrangement shown in Figure 1, some difiiculty may be experienced due to capacitive coupling phenomena through the interelectrode capacities of the modulating tubes 68, I0. This may 'be avoided by using neutralizing circuits such as described in my copending application Serial No. 8,447, filed February 2'7, 1935, now Patent No. 2,101,438 dated December '7, 1937.

Often, however, this objectionable capacitive phenomena is avoided by grounding the grids 64, 66 for radio frequency currents by means of the condensers 200, 202 of Figure 2. The condensers 200, 202 are adjusted so as to series tune the inductance of the leads to 64, 66 and main- I tain them at ground radio frequency potential. The remaining portions of Figure 2 have been shown more or less diagrammatically and correspond in general to the apparatus shown in Figure 1. Condenser 86 of Figure 2, however, is made relatively large so as to maintain one end of the loop 84 grounded. The oscillator of Figure 2 is shown Within the rectangle 204 and contains all of the apparatus within the dash lines 204 of Figure l. The output circuit 56 of Figure 2 is shown conductively connected to the output conductors 48, and the output is illustrated as being directly used in a short wave antenna 58. Also, an important difference of the scheme shown in Figure 2 resides in the fact that the high frequency input to the modulator tubes 60, I0 is applied to the tuned cathode circuit '52 through the conductors 2), H2 connected to the oscillator output conductors 48, 50 through the coupling condensers 2I4, 2E6. In the system shown in Figure 2, three hundred thousand cycle frequency variation was linearly produced about a carrier of a frequency of 200 megacycles per second. This is a particularly wide band of frequency modulation and was obtained without accompanying amplitude modulation. Although theoretically a small amount of amplitude varia tion should occur, it was found that the amplitude modulation was too small to be noticeable in practice.

In the system shown in Figure 3, the inductance of the line I4 is varied by coupling thereto the loop 300 which is tuned by means of condenser 302 and blocked off from plate potential by means of the coupling condenser 304. The in- Six ductance of theline I4 is opposed oraided, depending upon the relative phases of the potentials fed backand the potentials of the line wave, an extent dependent upon the amplitude of the potential'fed back. Ifdesired, of course, condenser 304 maybe omitted, in which case the cylinder i 6 would have to beinsulated from'groundrather than :grounded as shown. Modulating waves, as shown, are 'fed through the transformer $00 to the grids '64, 66 which are grounded for radio frequency currents by means of bypass condensers 200, 202. As in Figure 2, the voltage feed back forfrequency modulation purposes is fed into the cathode circuit E2 of the modulator tubes .68, to through the transmission line .3). Output enery of Figure 3 may be fed to a directive antenna 3l2 connected to the output conductors 48, 50 through the coupling condensers 3M, 3H5.

At this point, attention may be called to the fact that one particular advantage of the foregoing system of frequency modulation is that the tuning of the coupling tube circuit, that is .to say, the modulation tube circuit, may be made as broad as needed without adversely affecting the .power factor of the frequency controlling circuit l4, I6. In addition, the power which coupling tubes '88, 1.0 are required to handle is also very smallas they are only required to affect a very small portion of the total oscillating power in the frequency controlling circuit, in the case shown, the line controlled system M, 6. Since tuning of the stud circuit increases the impedance of said circuit and results in high mutual energy transfer from stud to line and from line to stud, due to enhanced unity power factor feed conditions, this stud circuit is often operated somewhat out of tune with the frequency of the system.

In Figures 1 to 3 the capacity or inductance-of the frequency controlling circuit for an oscillation generator was varied in order to vary the frequency of oscillation in accordance with the modulating waves. In the scheme shown in Fig ure 4, both the capacity and inductance of a frequency controlling line are varied by the phase and amplitude controlled feed back.

For a better understanding of the system, shown in Figure 4, reference is made to the schematic diagram shown in Figure 5. Turning to Figure 5, the generator G delivers power to'the two tuned circuits C1, Ll, and C2, L2. These two circuits are coupled to each other and have a mutual inductance M. By means of the switch connected as shown the phase of one of the circuits can be reversed with respect to the other. Because of this reversal of phase and due to the fact that the circuits have mutual reactance, the natural .period of the combination will be different for the two positions of the switch and will also vary with respect to the amount of current drawn by each circuit.

With the foregoing principles in mind, the system of Figure 4 may readily be understood. The oscillator 500 of Figure 5 is frequency controlled by a frequency controlling resonant transmission line 502 one half wave length long. The oscillator 500 is more fully described in my copending application Serial No. 6,814, filed February 15, 1935 now Patent No. 2,095,990, dated October 19.

1937. A portion of the oscillator output is fed through the loosely coupled pickup coil to the vacuum tube switching circuit 505 which replaces the switch S of Figure 5. The vacuum tube switching circuit 506 is supplied with modulating 'waves through the modulation transformer 1&0

so as to alternately render tubes 508 or 5E0 conductive, in this way reversing the feedback to the half wave line 504 which is coupled to the line 502. It is to be noted that line 500 corresponds 'to circuit C1, L1 of Figure 5, and line 502 to the circuit C2, L2 of Figure 5 and that the switch S of Figure 5 has been replaced in Figure 4 by the tube arrangement 506.

The two pairs of tubes 508, 5l0 form two pushpull coupling stages in which the phase of the 1 grids in one pair is opposite to the phase in the gridsof the other pair. As before explained, the plates are connected to a deflector or auxiliary line 504.

When there is no modulation potential in the transformer 100, the system is balanced so that the generator control line receives no energy from the four plates of the tube switching circuit 506. When, however, modulation is supplied, the tubes in which the grids are made more positive will provide a greater coupling than those which are made negative, as a result of which the effect upon the oscillator frequency controlling line 502 is the same as if the reflector 504 had been moved back and forth with respect to it, or is the same as if the inductance and capacity of the line 502 had been simultaneously varied. Preferably, the lines 504, 502 are housed within a shielded compartment or within a metallic cylinder and temperature controlled.

In the prior arrangements, the modulating potentials are applied to the control grids or to the cathodes of the coupling or control tubes between the generator and the frequency control line or circuit. Of course, I contemplate the use of a coupling tube or coupling tubes wherein the control potentials are applied to electrodes of the tube or'tubes other than the control grid and cathode. For example, the controlling potentials may be applied to the anodes of a pair of coupling or modulator tubes 68, 10 in phase displaced relation from any source in any manner. For example, the modulating potentials may be applied in phase displaced relation by way of a transformer I00 connected as shown in Figure 6. control grids of these tubes may be coupled by way of a line 603 terminating in an inductance 606 coupled to an inductance 608 connected between the anodes of the tubes in the oscillator. The oscillator may utilize the principles disclosed in connection with the-prior modifications. All that is essential is that the control electrodes of the oscillator be connected with loops I0 and I2 coupled as in the prior modifications to the line The anodes of the coupling tubes 68 and 10 may be coupled by way of coupling and blocking condensers 600 and G0! and a tuned non-symmetrical circuit NC to an element 92 terminating in .a plate 94 located in the field of the line I 4. The circuit NC may be tuned to the mean operating frequency of the oscillator as determined by the line l4, Hi, i. e., to the frequency of said oscillator and line when no modulating potentials are applied.

In operation, as in the prior modifications, high frequency energy is supplied from the oscillator by way of line iiiis to the control grids of tubes 68 and 10. These tubes in turn supply energy to the circuit NC and from said circuit to the element 9 2. The amplitude and phase of the energy supplied to the element 94 vary in accordance with the amplitude and phase of the modulating potentials, thus producing in the field of M energy the phase of which opposes 0r aids the energy of Thesaid field. Plate modulation as thus described is preferable in many cases since it is easy to obtain. Moreover, it has been found that the modulation produced in this manner is substantially linear.

The tendency of the push-push potentials in the output of the tubes 68 and H; to react on the energy in the push-pull circuit between the grids of tubes 63 and iii may be eliminated by neutralizing the balanced circuit as described more in detail in connection with my U. S. application Ser. No. 8,447, filed February 27, 1935, now Patent No. 2,101,438 dated December '7, 1937. Preferably this is accomplished by connecting the control grid of each of the tubes 68 and 18 to ground by way of condensers K and an inductance Y. For push-push feedback potentials from the nonsymmetrical circuit NO, the two condensers are in parallel as are the capacities of tubes 68 and i0 and. form in conjunction with the inductance Y a tuned short circuit for the operating frequencies between each grid and ground, thus establishing the aforementioned balance or neutralizing effect. For push-pull feed-forward, that is from line 803 to the non-symmetrical control circuit NC, the inductance Y is in a neutral zone and does not form a part of the input alternating current circuit.

The frequency modulated energy may be derived from a circuit coupled to the cathodes of the oscillator tubes, as disclosed in Figure 1, or from a circuit coupled to the anodes of the tubes, as disclosed in Figure 6.

In the arrangement shown in Figure 7, a single oscillator tube H153 has its control grid coupled as shown to a point on the line M which may be moved. The output of the oscillator TUB may be connected in a tuned circuit ml coupled to the control grid of a modulator or controlling tube F02. The anode of the tube 162 may be connected to a tuned circuit Hi l which is in turn coupled to the elements 92, 94. Any tendency of the coupling or modulator tube I62 to oscillate or operate at a frequency other than the frequency applied from lflil may be neutralized by connecting the neutralizing condenser NO between the anode circuit FM and the control grid of 102. Plate modulation may be utilized here. Preferably, the anode electrode of the modulator tube is connected as shown to a point on the inductance of the circuit 1%. Anode potential may be supplied to both tubes from a rectifier by way of a radio frequency choke RFC. Any radio frequency potentials appearing in the anode circuits of 79:2 and H0 may be shunted around the rectifier by a bypassing condenser connected as shown.

Here, as in the prior modifications, the field of I4 is aided or opposed by high frequency energy of a phase and amplitude determined by the phase and amplitude of the modulating potentials applied to the anode of tube 182, and it is preferable to operate circuit HM slightly out of tune.

I have found in practice-that when using extremely wide band modulation, power Variations or modulations of the frequency produced in the oscillator may be caused by said power variations. Since the frequency of line controlled oscillators is only slightly affected by plate voltage variations, the simplest way of compensating for the power variations is to vary the plate voltage of the oscillator in accordance with the modulation.

In Figure 8, I have shown for purposes of illustration a frequency modulator of the type disclosed hereinbefore in which additional means is provided for compensating any tendency of modulation in the oscillator due to wide band modulation power variations. The frequency control line l4, I6 of a quarter-wave length and the oscillator 800, 802 and their circuits are somewhat similar to the corresponding elements of Figure 1. Here, however, a plate modulating arrangement similar in some respects to Figure 6, and comprising tubes 804, 886 coupled on the one hand by a line 803 to the anode circuit 835 of the oscillator tubes is used. The anodes of the tubes 804, 806 are coupled by blocking condensers 809 as shown, to a non-symmetrical circuit BIO, in turn coupled to the elements 92, 94. Modulating potentials are supplied to the anodes of the tubes 804 and 806 from the secondary winding of transformer M4, the primary winding of which is in the anode circuit of a modulator tube 8H2. The control grid of the modulator tube may be coupled by way of a transformer I00 to any source of modulating potential. Here, as in the modification illustrated in Figure 6, the phase and amplitude of the energy produced by 94 in the field of M vary in accordance with the phase and amplitude of the modulating potentials supplied by the transformer I00. As indicated above, undesired modulations of the oscillations produced in the oscillator and line l4, it may occur when extremely wide band modulation power is utilized. To overcome this, a compensating modulator tube 856 has its control grid connected as shown with the secondary winding of transformer l3!) and its anode connected as shown to the circuit 895 between the anodes of the modulator stage.

The anode of the compensation modulator tube 8| 6 and the anodes of the oscillator tubes 800 and 802 are connected as shown by way of a low frequency reactor to the positive terminal of a source of potential, such as a rectifier, the negative terminal of which is connected to the cathodes of the tubes M6, 800 and 892. In this manner, the direct current potentials supplied to the anodes of the oscillator tubes 8% and 892 are controlled to the necessary extent by the modulating potentials to thereby compensate for the variations in power occurring in the oscillators when a wide band of frequency modulation is accomplished. If this compensation is not used, too much off tuning of the stud circuits will have to be used, and this will render the device less eificient.

Radio frequency oscillations may be prevented from reaching the modulating circuits from the non-symmetrical circuit 8W by radio frequency choking inductances connected as shown between the anodes of 804 and 806 and the secondary winding of Bl. By-passing condensers may be connected as shown between the choking inductances and ground. Direct current potentials from the electrodes of the modulator tubes, and the amplifier, may be supplied from sources connected as shown.

In the modification shown in Figure 8, the frequency modulated carrier wave may be derived by way of a circuit coupled to the anode circuit of the oscillator tubes 80!], 862 or by way of a circuit coupled to the cathode circuits of the oscillator tubes or by way of circuits coupled to the anode circuit and to the cathode circuit of the oscillator tubes.

Having thus described my invention, what I claim is:

1. A frequency modulation system comprising an oscillator having an output circuit and a frequency controlling circuit, means for applying voltage from said output circuit to said frequency controlling circuit, andmeans responsive to signaling waves for reversing the phase'and varying the amplitude of the applied voltage thereby varying the frequency of oscillations generated by said oscillator.

2. Apparatus as claimed in the preceding claim, characterized by the fact that said output circuit is tuned.

3. In a frequency modulation system, an oscillation generator having a frequency controlling circuit and an output circuit, a capacitive element associated with said frequency controlling circuit, means for feeding back voltages from said output circuit to said capacitive element, and means responsive to signaling waves for reversing the phase and varying the amplitude of the voltages fed to said capacitive element whereby the effective capacitance of said frequency controlling circuit is varied, thereby varying the frequency of the energy appearing in said output circuit.

4. In a frequency modulation system, an oscillation generator having a frequency controlling circuit and an output circuit, a reactive element associated with said frequency controlling circuit, means for feeding back voltages from said output circuit to said reactive element, and means responsive to signaling waves for reversing the phase and varying the amplitude of the voltages fed to said reactive element whereby the effective reactance of said frequency controlling circuit is varied, thereby varying the frequency of the energy appearing in said output circuit.

5. In a frequency modulation system, an oscillation generator having a frequency controlling circuit and an output circuit, an inductive element associated with said frequency controlling circuit, means for feeding back voltages from said output circuit to said inductive element, and means responsive to signalling waves for reversing the phase and varying. the amplitude of the voltages fed to said inductive element whereby the effective inductance of said frequency controlling circuit is varied, thereby varying the frequency of the energy appearing in said output circuit.

6. In a frequency modulation system, an oscillator having an output circuit and a frequency controlling circuit consisting of a resonant line, capacitive means for applying voltage from said output circuit to said frequency controlling line, and means responsive to signalling waves for varying the phase and amplitude of the voltage applied from said output circuit to said frequency controlling line, thereby varying the effective capacitance of said line and consequently varying the frequency of oscillations generated by said oscillator.

7. In a. frequency modulation system, an oscillator having an output circuit and a frequency controlling circuit consisting of a resonant line, inductive means for applying voltage from said output circuit to said frequency controlling line, and means responsive to signalling waves for varying the phase and amplitude of the voltage applied from said output circuit to said frequency controlling line, thereby varying the effective inductance of said line and consequently varying the frequency of oscillations generated by said oscillator.

8. In a frequency modulation system, an oscillator having an output circuit and a frequency controlling circuit consisting of a resonant line, reactive means for applying voltage from said output circuit to said frequency controlling line and means responsive to signalling waves for varying the phase and amplitude of the voltage applied from said output circuit to said line, thereby varying the effective re-actance of said line and consequently varying'th-e frequency of oscillations generated by said oscillator.

9. In a frequency modulator an oscillator having an output circuit, an additional circuit coupled at its input to the output circuit of said oscillator, said additional circuit being coupled at its output to said oscillator, phase reversing means in said additional circuit, and means for controlling said phase reversing means at signal frequency.

10. In a frequency modulator an oscillator having an input circuit and an output circuit, an additional circuit coupled at its input to the output circuit of said oscillator, said additional circuit being coupled at its output to the input circuit of the oscillator, phase reversing means in said additional circuit and means for controlling said phase reversing means at signal frequency.

11. In a frequency modulator, a thermionic oscillator having a frequency determining circuit, an additional circuit coupled at its input to said oscillator, said addidtional circuit being coupled at its output to said frequency determining circuit, phase reversing means in said additional circuit, and means coupled to said phase reversing means for controlling the same at signal frequency.

12. In signalling systems, an oscillator circuit, frequency control means coupled to said circuit, a coupling circuit coupled at its input to said oscillator circuit and at its output to said frequency control means, and means in said coupling circuit for reversing the phase of the oscillations therein at signal frequency, said coupling circuit being tuned substantially to the frequency to which said oscillator circuit is to operate.

13. In a signalling system, an oscillator circuit, frequency determining means coupled thereto, a coupling circuit coupled at its input to said oscillator and at its output to said frequency determining means, said coupling circuit being tuned at its output to a frequency slightly different than the operating frequency of said oscillator, and thermionic means in said coupling circuit for reversing the phase of the oscillations passed thereby at signal frequency.

14. In a frequency modulator an oscillator of the electron discharge device type having input and output electrodes coupled in oscillation producing circuits, frequency stabilizing means including linear adjacent conductors in one of said circuits, and additional electron discharge system having input electrodes coupled to one of said oscillation producing circuits and having output electrodes coupled to one of said oscillation producing circuits, means for varying the impedance i of said additional electron system at signal frequency, and means for tuning said last named coupling to a frequency slightly different than the frequency to which said oscillation producing circuits are tuned.

15. In a frequency modulator, a tube oscillator having input and output electrodes coupled in oscillation producing circuits, 2. frequency controlling line of the concentric cylinder type in one of said circuits, an electron discharge tube system having input electrodes coupled to the output of said oscillator and having output electrodes capacatively coupled to one element of said frequency controlling line, and means for varying the frequency of operation of said tube oscillator comprising a source of modulating potentials coupled to said electron discharge system.

16. In a frequency modulating system, a pair of thermionic tubes, a frequency controlling element coupled with like electrodes in said tubes, an output circuit coupled with other like electrodes in said tubes, a pair of thermionic modulator tubes, a circuit coupling the input electrodes of said pair of thermionic modulator tubes in pushpull relation to the output circuit of said oscillator tubes, a push-push circuit coupling the output electrodes of said modulator tubes to said frequency controlling element, and a source of modulating potentials connected in phase opposition to like electrodes in said last named tubes.

17. In a frequency modulating system a pair of thermionic tubes, a frequency controlling element coupled with like electrodes in said tubes, an oscillation circuit coupled with other like electrodes in said tubes, a pair of thermionic modu lator tubes, a circuit coupling the input electrodes of said pair of thermionic modulator tubes in push-pull relation to said oscillation circuit, a push-push circuit coupling the output electrodes of said modulator tubes to said frequency controlling element, a source of modulating potentials connected in phase opposition to like electrodes in said last named tubes, and a coupling between said source of modulating potentials and the outputelectrodes of the said first named pair of tubes.

18. In a frequency modulating system a pair of thermionic tubes, a frequency controlling element coupled with the control electrodes in said tubes, an output circuit coupled with the output electrodes in said tubes, a. pair of thermionic modulator tubes, a circuit coupiing the input electrodes of said pair of thermionic modulator tubes in push-pull relation to the output circuit of said oscillator tubes, a reactive circuit coupling the output electrodes of said modulator tubes to said frequency controlling element, a source of modulating potentials connected in phase opposition to the output electrodes in said modulator tubes, and a coupling between said source of modulating potentials and the output electrodes of said first named pair of tubes.

19. In a frequency modulating system a pair of thermionic tubes, a frequency controlling element coupled with like electrodes in said tubes, an output circuit coupled with other like electrodes in said tubes, a pair of thermionic modulator tubes, a circuit coupling the cathodes of said pair of thermionic modulator tubes in push-pull relation to the output circuit of said oscillator tubes, a pushpush circuit coupling the output electrodes of said modulator tubes to said frequency controlling element, means for maintaining the control grids of said modulator tubes at ground radio frequency potential, and a source of modulating potentials connected in phase opposition to like electrodes in said last named tubes.

20. In a system for producing oscillations and modulating the frequency of the oscillations at signal frequency, an electron discharge tube having a control grid, a cathode and an anode, an output circuit connected between the anode and cathode, a frequency controlling circuit comprising concentric conductors the length of which is substantially one-quarter the length of the waves to be generated, means connecting one of said conductors to the cathode of said tube, means connecting the other of said conductors to the control grid of said tube, an additional electron discharge tube having its input electrodes coupled to said output circuit and its output electrodes connected in an output circuit, means coupling said last named output circuit to one of said concentric conductors, means for tuning said last named output circuit to a frequency different than the frequency of the oscillations generated in said first named tube and line, and means for modulating the impedance of said last named tube at signal frequency.

21. In a system for producing wave energy of carrier frequency and modulating the frequency of the same through a wide range of frequencies, an electron discharge device having a control grid, a cathode and an anode, a resonant line comprising concentric conductors one of which is of a length at least one-quarter the length of the waves to be generated, a tuned oscillatory circuit connected between the anode and cathode of said tube, a connection between the control grid of said tube and one of said conductors, a connection between the cathode of said tube and the other of said conductors, an additional electron discharge device having a control grid, a cathode and. an anode, a circuit connecting the control grid and cathode of said last named tube to said output circuit, an output circuit connected between the anode and cathode of said additional tube, means for tuning said last named output circuit to a frequency different than the frequency of the wave generated by said first named tube, means for coupling said last named output circuit to one of said conductors, means for varying the impedance of said last named tube at signal frequency, and means connected with said last named means for controlling the impedance of said first named tube at signal frequency.

22. In a frequency modulation system, an osciilation circuit including adjacent conductors, means for diverting oscillatory voltage from said circuit, means for reversing the phase and varying the amplitude of the diverted voltage at signal frequency, an element adjacent one of said conductors and energized thereby, and means for impressing the diverted phase reversed and amplitude varied voltage on said element to I phase or in phase displaced relation relative to the voltage on said conductor, and means connected with said last named means for reversing the phase of said impressed voltage at signal frequency.

24. A system as recited in claim 23 wherein said one of said conductors is connected by a high impedance to said other of said conductors and in which said last named means operates on said high impedance connecting said conductors.

25. In a frequency modulation system, an electron discharge device having input and output electrodes connected in oscillation producing circuits, a frequency controlling line comprising concentric conductors, one of which is connected to the cathode of said device and the other of which is connected to another electrode of said device, a conductive element coupled to one of said conductors and connected to the other of said conductors by a high impedance, means for i in applying voltage from one of said circuits connected with said electron discharge device to said conductive element, and means for reversing the phase of the voltage applied to said conductive element in accordance with signals to thereby vary the tune of said line and consequently the frequency of the oscillation generator by said device.

26. A system as recited in claim 25 wherein said last named means also varies the amplitude of the voltage applied to said conductive element in accordance with said signals.

27. A system as recited in claim 23 wherein said last named means also varies the amplitude of the voltages impressed on said one of said conductors in accordance with signals.

28. In a frequency modulation system, an ultrahigh frequency oscillator, frequency control means operably associated with said ultra high frequency oscillatory to control the frequency of oscillation thereof, means for deriving oscillatory energy produced by said oscillator, means for feeding said derived oscillatory energy to said frequency control means, and means operable at signal frequency for shifting the phase of the energy fed to said frequency control means back and forth between two predetermined limits to control the frequency of oscillation of said oscillator in accordance with said signals.

29. In a frequency modulation system, an ultrahigh frequency oscillator of the electron discharge device type having input and output electrodes coupled in oscillation producing circuits, frequency control means operably associated with said ultrahigh frequency oscillator to control the frequency of oscillation thereof, electron discharge tube means for deriving oscillatory energy from said oscillator, means for feeding said derived oscillatory energy from said electron discharge tube means to said frequency control means, and means for controlling the impedance of said electron discharge tube in accordance with signals for shifting the phase of the energy fed to said frequency control means back and forth between two predetermined limits to control the frequency of oscillation of said oscillator in accordance with said signals.

30. In a frequency modulation system, an oscillation generator including electron discharge devices having input and output electrodes connected in oscillation producing circuits, a frequency control means in the form of concentric lines coupled with one of said circuits, and means for modulating the frequency of the oscillations produced comprising a pair of electron discharge tubes having input electrodes coupled in push-pull relation with one of said oscillation producing circuits, said electron discharge tubes having output electrodes, a circuit of substantial impedance to the oscillations generated coupling the output electrodes of said tubes to one of said concentric lines, a source of modulating potentials and circuits for impressing said modulating potentials in phase opposition on the output electrodes of said discharge tubes.

31. In a frequency modulation system, an electron discharge device having input and output electrodes coupled in oscillation producing circuits, frequency controlling means in one of said circuits including concentric lines, and means for controlling the reactance of one of said lines at signal frequency to thereby control the frequency of the oscillations produced at signal frequency including an electron discharge tube having input electrodes coupled to one of said circuits, said tube having output electrodes, a tuned circuit of material impedance to the oscillations produced in said device and circuits coupling the output electrodes of said tube to one of said lines, a modulator tube having output electrodes connected in shunt to the electrodes in said tube, and a source of modulating potentials for varying the impedance of said modulator tube at signal frequency.

NILS E. LINDEN'BLAD. 

